Co-use of tobacco and cannabis has been increasing, especially among young adults in the US, with more than 20% of young adults aged 18-24 reporting use of both substances in the past month. There is growing evidence that co-use of these substances increases the health consequences and problematic psychosocial outcomes of either alone, and that use of one product increases the probability of use and failure to stop the other. Co-use is a broad term and may encompass using both substances within a given time frame (e.g., past month), within a given episode (concurrent use), in sequence (chasing), or mixed together in the same delivery device (co- administration). Until recently, co-use was also generally limited to combustible methods, either smoking each product separately or combining them (e.g., in blunts or spliffs). With the increase in electronic nicotine delivery systems (ENDS) as well as the increase in states legalizing recreational cannabis, there are new options for co- use, and ENDS devices that vape e-liquids containing nicotine are becoming more popular methods to consume cannabis. Although research on co-use is growing, there have been numerous methodological limitations of prior work, including a lack of detailed data about differences in specific patterns of co-use and modes of delivery, especially given the rising popularity of alternative tobacco products, as well as a reliance on survey data. Less is known about the factors that may contribute to the different patterns of co-use in young adults, that may maintain co-use, and that may be associated with different outcomes. The overall goal of this study is to examine the situational, momentary factors associated with the different patterns of tobacco and cannabis co-use, how they may vary by delivery mode, and how they are associated with changes in behavioral outcomes (e.g., dependence, changes in rates of use, problem behaviors). This project will use ecological momentary assessment (EMA) to gather in-depth, real-time naturalistic reports of 425 young adult (aged 18-25) tobacco and cannabis co-users? daily experiences and product use behaviors. Across two waves of EMA, we will capture episodes in which young adults use each product alone as well as when they use them concurrently (during the same episode), either in a common delivery device (e.g., blunt or vaporizer) or in succession. We will examine how the proximal context in which the combination of product use occurs and the individual's subjective, affective, and neurocognitive reactions to those use experiences vary by delivery mode (combustible or vaporized) and individual characteristics, and how these factors influence future tobacco and cannabis use patterns. Knowledge gained will provide needed evidence about the behavioral and neurocognitive consequences of co-use use that will help better inform prevention, intervention, and policy approaches to reducing harms of both products. This project is also timely in that its data collection occurs in a state (Illinois) that will have just legalized recreational cannabis use (effective January, 2020).